1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent (EL) display device and a method of fabricating the same.
2. Discussion of the Related Art
An organic electroluminescent (EL) display device, which is a type of flat panel display, is a self-emission type display. In general, the organic EL display device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Accordingly, the organic EL display device does not require an additional light source and has a light weight, thin profile, and compact size.
The organic EL display device also has other excellent characteristics such as low power consumption, superior brightness, fast response time and simple fabrication process. As a result, the organic EL display device is regarded as a promising display for next-generation consumer electronic applications, such as cellular phones, car navigation systems (CNS), personal digital assistants (PDA), camcorders, and palmtop computers.
There are two types of organic EL display devices: passive matrix type and active matrix type. While both the passive matrix organic EL display device and the active matrix organic EL display device have simple structures and are formed by a simple fabricating process, the passive matrix organic EL display device requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic EL display device is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic EL display device decreases. In contrast, active matrix organic EL display devices are highly efficient and can produce a high-quality image for a large display with a relatively low power.
FIG. 1 is a schematic cross-sectional view of an organic electroluminescent display device according to the related art. In FIG. 1, an organic EL display device 10 includes first and second substrates 12 and 28 attached to each other by a sealant 26 with a space therebetween. An array element layer 14 is formed on the first substrate 12 and includes a thin film transistor (TFT) T. In addition, a first electrode 16, an organic luminescent layer 18 and a second electrode 20 are formed on the array element layer 14. The first electrode 16 is connected to the TFT T. The organic EL layer 18 may separately display red, green, and blue colors in each pixel region P.
The organic EL display device 10 is encapsulated by attaching the first substrate 12 to the second substrate 28. The second substrate 28 includes a moisture absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the absorbent material 22 and the filled absorbent material 22 is fixed by a holding element 25.
FIG. 2 is a schematic circuit diagram of an array layer of an organic electroluminescent display device according to the related art. In FIG. 2, a gate line 32 is formed along a first direction on a transparent insulating substrate 12, and a data line 34 is formed along a second direction intersected with the gate line 32, thereby defining a pixel region P. A power line 35 also is formed along the second direction and spaced apart from the data line 34. An insulating layer (not shown) is interposed between the gate line 32 and the data line 34.
In addition, a switching element TS is formed in the pixel region P. The switching element TS includes a switching gate electrode 36, the switching active layer 40, a switching source electrode 46 and a switching drain electrode 50. Further, a driving element TD electrically connects the switching element TS. The driving element TD includes a driving gate electrode 38, a driving active layer 42, a driving source electrode 48 and a driving drain electrode 52. In particular, the switching gate electrode 36 is connected to the gate line 32, the switching source electrode 46 is connected to the data line 34, and the switching drain electrode 50 is connected to the driving gate electrode 38 through a first contact hole 54. The driving source electrode 48 is connected to the power line 35 through a second contact hole 56. In addition, the driving drain electrode 52 is connected to a first electrode 16 in the pixel region P. The power line 35 overlaps a first capacitor electrode 15 with the insulating layer interposed therebetween to form a storage capacitor CST.
FIG. 3 is a schematic plan view of an organic electroluminescent display device according to the related art. In FIG. 3, a substrate 12 includes a data pad region “E” along a first side and a gate pad region “F” along a second side adjacent to the first side. A power input pad 70 is formed at an edge portion of the substrate 12. The power input pad 70 is connected to a power line (not shown) and an electric power is supplied to a driving element through the power input terminal 70 and the power line. In addition, a ground line 60 is formed along third and fourth sides of the substrate 12. The ground line 60 is connected to a second electrode (not shown) and a common voltage is supplied to the second electrode through the ground line 60.
FIG. 4A is a schematic cross-sectional view along IVa-IVa of FIG. 2, and FIG. 4B is a schematic cross-sectional view along IVb-IVb of FIG. 3. In FIGS. 4A and 4B, a driving thin film transistor (TFT) TD including a driving gate electrode 38, a driving active layer 42, a driving source electrode 56 and a driving drain electrode 52 is formed on a substrate 12. An insulating layer 57 is formed on the driving TFT TD and a first electrode 16 connected to the driving drain electrode 52 is formed on the insulating layer 57. An organic luminescent layer 18 for emitting light of a specific color is formed on the first electrode 16 and a second electrode 20 is formed on the organic luminescent layer 18.
In addition, a storage capacitor is formed to be electrically parallel to the driving TFT TD and includes first and second capacitor electrodes 15 and 35, wherein a portion of a power line overlapping the first capacitor electrode 15 is used as the second capacitor electrode 35. The second capacitor electrode 35 is connected to the driving source electrode 56 and the second electrode 20 is formed on an entire surface of the substrate 12 having the driving TFT TD, the storage capacitor, and the organic luminescent layer 18.
A ground line 60 is formed in a periphery of the substrate 12. The ground line 60 is formed of the same material as the driving source electrode 56, the driving drain electrode 52 and the power line 35. In addition, the ground line 60 is connected to the second electrode 20 through a plurality of contact holes 52 and a common voltage is, supplied to the second electrode 20 through the ground line 60. Since the second electrode 20 is formed through an evaporation method using a shadow mask, the second electrode 20 is porous and a poor hardness. Moreover, the ground line 60 and the second electrode 20 are individually formed by independent processes. Thus, the ground line 60 is not simultaneously formed with the second electrode 20.
The organic luminescent layer 18 between the first and second electrodes 16 and 20 may be formed of a single layer or a multiple layer. The organic luminescent layer 18 of a multiple layer includes an emission layer 18a, a hole transporting layer (HTL) 18b, a hole injecting layer (HIL) 18c, an electron transporting layer (ETL) 18d and an electron injecting layer (EIL) 18e. The HTL 18b and the HIL 18c are interposed between the first electrode 16 and the emission layer 18a, and the ETL 18d and the EIL 18e are interposed between the second electrode 20 and the emission layer 18a. The HIL 18c and the EIL 18e shift a Fermi level, thereby moving holes and electrons easily.
However, since the EIL 18e and the second electrode 20 are formed by one shadow mask, the EIL 18e is interposed between the ground line 60 and the second electrode 20. In particular, the EIL 18e is formed of one of fluoride compound and oxide compound, such as LiF and Li2O2. As a result, the EIL 18e is not conductive and functions as a resistor between the second electrode 20 and the ground line 60. Accordingly, heat is generated between the second electrode 20 and the ground line 60 because the EIL 18e functions as a resistor. Thus, the organic electroluminescent display device according to the related art deteriorates faster due to heat, thereby shortening product life span.